vOptimizer does more than optimize for virtual machine size alone, as you can read here with full documentation available here.

I don't have the need for too many virtual machines, and ran the following optimizations. Each took around 20 minutes to complete, taking all the default Windows optimization options, which seem pretty smart to me. This is what I got:

The base #1 VM had little more than Windows XP and Microsoft Office 2003 installed. The other XP VMs had various versions of IBM Lotus Notes and Domino installed, with somewhat varying numbers and sizes of Lotus Notes databases. The Vista Business VM had Office 2003 and various other bits and pieces installed on top of it.

As you can see, vOptimizer gave pretty good results indeed, in terms of disk space optimization. In a production environment having maybe hundreds or even thousands of VMs running, it looks like you would save major amounts of space, helping to reduce data center sprawl and certainly yielding power savings.

I cannot comment in a fair manner on the effectiveness of the other optimizations that vOptimizer carries out, since my test VMs were used in a very lightly loaded manner. However, I would expect that there should be some useful performance gains, even if the percentage improvement in system performance isn't as great as the percentage disk space reduction (and I don't know, one way or the other).

So there you are, some real test results. As the saying goes, your mileage may vary, but you can download a trial copy of vOptimizer and be able to carry out a handful of test optimizations for your own environment.